A voltage controlled oscillator (VCO) is a component that can be used to translate DC voltage into a radio frequency (RF) voltage. The magnitude of the output signal is dependent on the design of the VCO circuit and the frequency of operation is determined by a resonator that provides an input signal. Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock signal from an external reference or from an incoming data stream. VCOs are therefore often critical to the performance of PLLs. In turn, PLLs are typically essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying information so that the information can be used for its intended purpose. PLLs are particularly important in wireless networks as they enable the communications equipment to quickly lock onto the carrier frequency onto which communications are transmitted.
FIG. 1 illustrates a simplified diagram of a voltage controlled oscillator 100 (“VCO”). The illustrated VCO 100 includes a transistor 102 and two feedback capacitors 104 (connected across the transistor's base and emitter) and 106 (connected between the transistor's emitter and ground), respectively. A resonator 108 is coupled to the base terminal of the transistor 102 through capacitor 110. A tuning network 112 is coupled to the resonator 108 through capacitor 114 and is coupled to the base terminal of the transistor 102 through capacitor 114 and capacitor 110. The emitter terminal of the transistor 102 is grounded through inductor 116 and resistor 118. The collector terminal of the transistor 102 is biased through inductor 120 and resistor 122 by DC voltage supply 124 (“Vcc”). The collector terminal of the transistor 102 is connected to the base terminal of the transistor 102 through inductor 120 and resistor 126. The base terminal of the transistor 102 is connected to ground through resistor 128. An output terminal 142 is coupled to the collector of the transistor through capacitor 144.
The illustrated tuning network 112 includes a capacitor 130, a tuning voltage source 132, a varactor 134, a pair of inductors 136, 138 and a diode 140 connected as indicated. The tuning network 112 is adapted to provide a voltage variable reactance that tunes oscillation to a desired frequency.
Non-linear, time varying (NLTV) frequency modulation may be generated due to changes in the transconductance and junction capacitance (Cbc, Cbe, and Cce) of the transistor 102 over the tuning range and due to variations in the bias point, operating temperature, operating frequency, oscillator conduction angle, and drive level. By selecting an appropriate transistor 102 and optimizing the values of feedback capacitors (104 and 106), phase noise might be reduced. However, such a reduction in phase noise is generally limited to a single fixed frequency and that frequency is generally not user-definable. Furthermore, the illustrated circuit is not user-definable for different operating frequencies (different resonator length is needed for corresponding resonance frequency).
In addition, junction (active device) modulation can create mixing products at new frequencies, which results in phase noise in the oscillator's spectrum. Moreover, the junction capacitance of the transistor (bipolar/FETs) typically changes in a non-linear manner. More particularly, the base-to-collector capacitance of the transistor (bipolar) can vary depending on the voltage across the base-to-collector junction, and is typically independent of the current flowing through the device. Thus, the collector-base junctions in the VCO are at different values of capacitance during an RF oscillation cycle.
A phase hit can be defined as a random, sudden, uncontrolled change in the phase of the signal source that typically lasts for fractions of a second. It can be caused by temperature changes from dissimilar metals expanding and contracting at different rates, as well as from vibration or impact. Microphonics, which are acoustic vibrations that traverse an oscillator package and circuits, can cause a change in phase and frequency. Microphonics are usually dealt with using a hybrid resonance mode in a distributed (micro/strip-line, stripline, suspended stripline) medium.
Phase hits are typically infrequent. But they can cause signal degradation in high-performance communication systems. The effect of phase hits generally increases with data rate. If a phase hit cannot be absorbed by a device (e.g., a receiver) in a communication system, a link may fail resulting in a data loss. As a result, a continuing task is reducing or eliminating phase hits. While phase hits have plagued communication equipment for years, today's higher transmission speeds tend to accentuate the problem given the greater amount of data that may be lost as a result of a phase hit.
The resonator 108 may include LC resonators, ceramic, cavity resonators, dielectric resonators, sapphire-loaded cavity (SLC) resonators, bulk acoustic wave (BAW) resonators, optoelectronic (OE) resonators, yttrium iron garnet (YIG) resonators, Radio Frequency-Micro Electro Mechanical System (RF-MEMS) and planar resonators.
LC resonators are typically formed with a plurality of inductors and capacitors and have a low Q factor. A perfectly lossless resonant circuit generally operates as an oscillator, but truly lossless elements are difficult to realize. The performance of VCOs comprising integrated LC (inductor/capacitor) resonators generally suffer owing to the low quality-factor (Q) of the inductor used in the LC resonator tank. The LC resonator tank includes inductors and capacitors arranged to oscillate by exchanging current or voltage between inductors and capacitors with a finite frequency. Since loss resistance in the inductors and capacitors tends to dissipate energy in the oscillator, the LC resonator loses energy and eventually stops oscillating. Compensating for energy loss implied by the finite Q of the practical LC resonators with the energy supplying action of the active device (three terminal bipolar or FETs) in a finite time is one potentially attractive way to build a practical oscillator. However, even though the loss resistance may be compensated for in this manner by the active devices, the inherent loss resistance and noise associated with the active device (thermal, flicker, shot noise) still degrades the oscillation quality by introducing random jitter and noise, thereby, affects oscillation purity (oscillation amplitude and phase noise).
Additionally, the resonating structure changes and adjusts its response to correct and reduce the frequency variations when the frequency in the feedback loop varies. As the change in the resonator output response on a frequency variation becomes larger, the correction is also typically larger. The change in resonance frequency is a function of the Q value. A higher Q value indicates that the resonator is more insensitive to frequency variations and therefore the oscillator output frequency will be more stable. In order to provide oscillators with high spectral purity and stable outputs, the resonator should desirably have a high Q value as is required for a low noise oscillator. With respect to the LC resonator tank circuit, the quality factor is defined as the ratio of the energy stored in the LC resonator tank to the energy dissipated in the equivalent loss resistor per oscillation cycle. Thus, it is desirable that inductors in an LC tank oscillator have minimum resistance. Unfortunately, on-chip inductors generally have a high loss resistance due to the substrate resistance and resistance of the metal used. Thus, on-chip inductors typically have relatively low Q factors at microwave frequencies. Therefore, the phase noise performance of oscillators using on-chip inductors is generally poor and typically not suitable for modern wireless devices such as cellular phones or satellite communication equipment.
For example, a conventional microstripline resonator is typically etched on the circuit substrate, and is a metal cover is then applied over the circuit board. The capacitance between the planar microstripline section and the cover typically causes cover frequency shift effects. More specifically, the oscillator frequency is frequency modulated by microscopic movements of the cover caused by noise and vibration, thereby; creating microphonics effects that cause phase hits and appear as phase noise performance of the oscillators.
Cavity resonators typically are formed from conductive materials having a variety of possible shapes, including rectangles, cylinders, spheres, etc. Although cavity resonators can have high Q factors and low power requirements, they are typically impractical due to their large size.
Dielectric resonators typically are made of BaTi4O9 and ZrSnTiO4. These resonators typically resonate in various modes and exhibit high Q factors. However, dielectric resonators are typically costly and not very useful for applications requiring frequencies below 2 GHz. Sapphire-loaded cavity resonators (SLC) typically have high Q factors but can be costly.
Bulk acoustic wave (BAW) resonators are typically three-layer structures (e.g., top and bottom electrode layers of molybdenum sandwiching a middle layer of oriented piezoelectric aluminum nitride). BAW resonators typically have reasonably good Q factors. However, they are typically very sensitive to temperature and, therefore, usually require thermal stabilization in commercial applications.
Optoelectronic (OE) resonators use optic resonator systems that typically provide a high Q factor. However, the application of OE resonators is typically limited due to gaps in frequency coverage as well as relatively high spurs.
YIG resonators are made of yttrium iron garnet (Y3Fe5O11) and typically exhibit high Q factors. However, YIG resonators are typically limited to a narrow band of operating frequencies. Planar resonators (e.g., ring, hairpin, microstrip-spiral, coupled line resonators, etc.) can be implemented in integrated circuit fabrication processes, but are typically very large and have low Q factors.
RF-MEMS VCO technology is on the verge of revolutionizing wireless communication systems. RF-MEMS based VCO components such as inductors, variable capacitors, and resonators generally provide superior performance in terms of quality factor, noise, linearity, power consumption, size, and cost. Such performance generally cannot be achieved by conventional approaches. Thus, this makes the RF-MEMS VCO a prime candidate for wireless connectivity. However, MEMS devices, unlike ICs, contain fragile moving parts that must be properly packaged to meet specific requirements. These requirements include protection against handling, shielding against electromagnetic fields, near hermetic cavity seals, low temperature process, good heat-exchange, minimal thermal stress, and RF electrical feed through.
Ceramic and SAW (surface acoustic wave) resonators typically are costly parts that are not generally suited for fabrication by current integrated circuit (IC) technology. At present, a cellular transmitter can be implemented on a single IC chip, except for the ceramic or SAW stage resonators. Therefore, to reduce the transmitter's cost and make it more amenable for integration on an IC chip, it is desirable to eliminate the ceramic and SAW based resonators. One way to eliminate the ceramic or SAW resonator is to use a planar resonator. But planar resonators typically lack the required Q (quality factor) and therefore, limit the phase noise performance of an oscillator.
Reducing phase noise and realizing wideband tunability have been assumed to be opposing requirements due to the problem of the controlling loop parameters and the dynamic loaded Q of the resonator over wideband operation. The resistive losses, especially those in the inductors and varactors, are of major importance and determine the Q of a tank circuit. There have been several attempts to come to grips with these contradictory but desirable oscillator qualities. One way to improve the phase noise of an oscillator is to incorporate high quality resonator components such as surface acoustic wave (SAW) and ceramic resonator components. But these resonators are more prone to microphonics and phase hits. These resonators also typically have a limited tuning range to compensate for frequency drifts due to the variations in temperature and other parameters over the tuning range.
Ceramic resonator (CRO) based oscillators are widely used in wireless applications, since they typically feature very low phase noise at fixed frequencies up to about 4 GHz. CRO resonator-based oscillators are also known for providing a high Q and low phase noise performance. Typically, a ceramic coaxial resonator comprises a dielectric material formed as a rectangular prism with a coaxial opening running lengthwise through the prism and an electrical connector connected to one end. The outer and inner surfaces of the prism, with the exception of the end connected to the electrical connector and possibly the opposite end, are coated with a metal such as copper or silver. A device formed in this manner essentially comprises a resonant RF circuit, including capacitance, inductance and loss resistance that oscillates in a transverse electromagnetic (TEM) mode if loss resistance is compensated.
CRO oscillators, however, have some disadvantages, including a limited operating temperature range and a limited tuning range (which limits the amount of correction that can be made to compensate for the tolerances of other components in the oscillator circuit). CROs are also typically prone to phase hits (due to expanding and contracting at different rates with variation of the temperature for outer metallic body of the CRO and other components of the oscillator circuit).
In that regard, circuit designers sometimes consider designing a digitally implemented CRO oscillator to overcome the above problems, otherwise, large phase hits can occur. In addition, since the design of a new CRO oscillator is much like that of an integrated circuit (IC), development of an oscillator with a non-standard frequency requires non-recurring engineering (NRE) costs, in addition to the cost of the oscillators.
Due to the emergence of the mobile communications market, the need for a low cost, compact, and power efficient radio frequency (RF) circuit module is attracting considerable attention. The coexistence of second and third generation wireless systems generally requires multi-mode, multi-band, and multi-standard mobile communication systems. This, in turn, makes desirable a user-definable low cost and relatively high spectrally pure tunable signal source (e.g., voltage controlled oscillator (VCO) or oscillator). Mobile telephones and radios operating in several modes typically switch between receiving and transmitting frequencies, and, therefore, require low phase noise performance in each of the switched bands. Although separate VCOs may be used to switch between bands, this results in an increase in power consumption, the number of components, weight and cost, for example.
More specifically, mobile telephone receivers typically require the use of a VCO with a specific center frequency in order to process incoming signals. In mobile telephones that are adapted to operate in multiple modes, separate VCO circuits, each of which has a center frequency associated with one of the modes, are typically required to be present in the telephones in order to process received signals in the multiple modes. The use of multiple VCO circuits in multiple mode telephones increases the number of components required to implement such devices, and such circuits occupy valuable space on the circuit boards used for implementing the telephone. Using multiple self contained VCOs also has the disadvantage that it requires substantial circuitry, e.g., a complete set of self contained VCOs for the needed frequency band, each complete with a phase locked loop.
Thus, a need exists for methods and circuitry that overcome the foregoing difficulties, and improve the performance of an oscillator or oscillator circuitry, including the ability to absorb phase hits over the tuning range of operation. In addition, a need exists for VCOs that can reliably operate in multiple modes while avoiding the need for additional circuitry or multiple discrete VCOs.